Why didn't the skeleton want to make art anymore?
He didn't have the heart to put into it.
Skeleton Artist
Explanation
Explain Bear
Listen up, kid. The joke here is all about wordplay. Skeletons don't have hearts, right? And to "put your heart" into something means to care about it a lot. The skeleton couldn't care about the art, because he ain't got no heart! You're welcome, you dummy.